This invention relates generally to an inverter device having a plurality of switching elements turned on and off so that a dc voltage is converted to a three-phase ac voltage, and more particularly to a pulse width modulation (PWM) signal generating device for the inverter device provided for controlling on and off operations of each switching element in accordance with a time ratio so that a substantially sinusoidal output voltage is obtained.
One conventional pulse width modulation (PWM) signal generating circuit in an inverter device will be described. FIG. 18 schematically illustrates an arrangement of an inverter main circuit 10 of the inverter device. Six switching elements 13u, 13v, 13w, 13x, 13y and 13z are bridge-connected between main circuit bus lines 11 and 12 as well known in the art. Since either of the upper and the lower-switching elements of each of the three arms is turned on, these three switches turned on are referred to as reference symbols Sa, Sb and Sc respectively. The number of switching patterns is thus obtained as 2.sup.3 =8. Each phase voltage is .+-.V/2 with respect to a virtual neutral point. Where voltage space vectors correspond to three-phase instantaneous voltages in consideration of phase differences among the phases, each of the switching elements Sa, Sb, Sc is represented as "1" when the positive switching element 13u, 13v or 13w in each phase is turned on. Each of the switching elements Sa, Sb, Sc is represented as "0" when the negative switching elements 13x, 13y or 13z in each phase is turned on. Accordingly, each switching pattern can be represented by substituting "1" or "0" for each of Sa, Sb, Sc. As shown by solid lines in FIG. 19, these switching patterns can be represented as six fundamental voltage vectors out of phase from one another by an electrical angle of 2.pi./6 and two zero vectors (0, 0, 0) and (1, 1, 1).
FIG. 20 illustrates the three-phase PWM signal generating device for on-off control of the switching elements in the above-described inverter device. Phase command value classification means 14 classifies a phase command value .theta.* into unit regions obtained by dividing an electrical angle of 2.pi./6 into twelve equal parts, for example and calculates an lead angle in the unit region into which the phase command value is classified. The result of classification and the calculated lead angle are produced as 4-bit information. Switching pattern determining means 15 is provided for determining the switching patterns corresponding to two kinds of fundamental voltage vectors (voltage space vectors) nearest to the unit region to which the phase command value .theta.* classified by the phase command value classification means 14 belongs and the switching pattern corresponding to the zero vector. The fundamental voltage vectors are out of phase from each other by an electrical angle of .pi./3. The switching pattern determining means 15 further determines a sequence that these determined switching patterns are delivered to the base terminals of the switching elements 13u-13z. Generally, two kinds of fundamental voltage vectors of the phases nearest to the phase command value .theta.* as exemplified in FIG. 19 and the vectors (1, 0, 0), (1, 1, 0) and the zero vector are specified as the switching patterns in the example of FIG. 19. One of two kinds of zero vectors (1, 1, 1) and (0, 0, 0) is selected so that the number of the switching operations becomes the smallest in consideration of the previous switching patterns.
The voltage space vectors which can be outputted must lie inside a hexagon formed by linking the points of six fundamental voltage vectors in FIG. 19 where the voltage space vectors each having a predetermined magnitude and phase are outputted by controlling the fundamental voltage vectors out of phase from each other by an electrical angle of 2.pi./6 and the zero vector in accordance with the time ratio. Accordingly, the region of the modulation to the sinusoidal waves performed by the above-mentioned control of the fundamental voltage vectors and zero vector is limited to the inside of an inscribed circle of the hexagon. Consequently, polar coordinates can be employed to realize optional voltage space vectors in the limit region and the region of an electrical angle of .pi./6 can be described in consideration of the symmetry.
FIG. 21 shows enlarged fundamental voltage vectors (1, 0, 0), (1, 1, 0) and the zero vector. In order that the voltage space vector corresponding to a command voltage vector including the phase command value .theta.* and voltage command value V* is outputted, it is obvious that the following expression needs to be satisfied by the geometric analysis shown in FIG. 21: EQU V sin (.pi./6-.theta.):V sin (.pi./6+.theta.):1-V{sin (.pi./6-.theta.)+sin (.pi./6+.theta.)}=t.sub.1 :t.sub.2 :t.sub.0 ( 1)
where t.sub.1, t.sub.2 and t.sub.0 are output times of the fundamental voltage vectors (1, 0, 0), (1, 1, 0) and the zero vector and .theta. is a lead angle of the phase command value .theta.* in the unit region to which the phase command value .theta.* belongs.
Holding time calculating means 16 is provided for obtaining the output times t.sub.1, t.sub.2 as shown in FIG. 20. More specifically, data of the lead angle .theta. is supplied to ROM tables 17 and 18 from the phase command classification means 14 so that the value of sin (.pi./6+.theta.) corresponding to the supplied lead angle is obtained. The holding time t.sub.1 of the switching pattern corresponding to the fundamental voltage vector (1, 0, 0) is obtained by multiplying one control period T.sub.SW by the voltage command value V* and further by the value of sin (.pi./6-.theta.). In the same way, the holding time t.sub.2 of the switching pattern corresponding to the fundamental voltage vector (1, 1, 0) is obtained by multiplying one control period T.sub.SW by the voltage command value V* and further by the value of sin (.pi./6+.theta.). The holding time t.sub.0 of the switching pattern corresponding to the zero vector is obtained by subtracting these holding periods t.sub.1 and t.sub.2 from the one control period T.sub.SW. One control period refers to a period in which the switching patterns corresponding to the two kinds of fundamental voltage vectors and the zero vector respectively are sequentially produced once.
Timing means 19 comprises a presettable counter 20, a switch 21 and a D-type flip flop 22. The presettable counter 20 has a data input terminal DATA to which data of each of the holding times t.sub.0, t.sub.1, t.sub.2 is inputted in accordance with the switching of the switch 21. The presettable counter 20 also has a clock terminal CK to which clock signals fck are inputted. The switch 21 is switched to an output terminal of the holding time corresponding to the switching pattern determined by the switching pattern determining means 15 when timing of each of the holding times supplied to the presettable counter 20 is completed and then, the subsequent holding time data is inputted to the data input terminal DATA. The flip flop 22 holds the formation state of the corresponding switching pattern until the timing of each holding time is completed, whereby the switching elements 13u-13z of the inverter device are desirably controlled to be turned on and off.
FIG. 22 illustrates a locus of a vector .psi. representative of a magnetic flux induced in a three-phase induction motor M as a three-phase load when an output of the inverter main circuit 10 controlled by the above-described PWM signal generating device is supplied to the motor M. Reference symbol .psi.' designates a mean locus and figures a circular orbit. The magnetic flux vector .psi. is represented as the time integral of the voltage space vector. Since each fundamental voltage vector has a predetermined value, the direction of the magnetic flux vector .psi. is the same as that of each fundamental voltage vector and the magnitude of the vector .psi. is proportional to the holding time of each fundamental voltage vector.
A manner of determining the output sequence of the switching patterns by the switching pattern determining means 15 will now be described. The output holding times t.sub.1, t.sub.2 of the switching pattern corresponding to the two fundamental voltage vectors determined by the switching pattern determining means 15 are calculated by the holding time calculating circuit 16. The output sequence of the switching patterns is so determined that the zero vector is interposed between longer vectors, that is, the zero vector is positioned at the start or the end of the vector with the longer holding time in one control period T.sub.SW. Generally, since the holding time of the fundamental voltage vector with the phase nearest to the phase command value .theta.* becomes longer because of the method of calculating the holding time, the switching pattern corresponding to the fundamental voltage vector with the longer holding time can be determined at the stage of the classification by the phase command value classification means 14.
FIG. 22 shows the case where the phase command value .theta.* is in the condition shown in FIG. 21. The phase command value .theta.* when t.sub.1 &lt;t.sub.2 is nearest to the fundamental voltage vector (1, 1, 0) where the holding times of the switching patterns corresponding to the fundamental voltage vectors (1, 0, 0), (1, 1, 0) and the zero vector are represented as t.sub.1, t.sub.2, t.sub.0, respectively. Accordingly, the switch 21 is switched so that the sequence of . . . t.sub.1, t.sub.2, t.sub.0, t.sub.0, t.sub.2, t.sub.1 . . . is repeated.
The above example will be described in more detail. When the switching pattern corresponding to the fundamental voltage vector (1, 0, 0) is held for the time t.sub.1 in the previous control period, the switching pattern corresponding to the fundamental voltage vector (1, 0, 0) is held for the time t.sub.1 in the following control period and then, the switching pattern corresponding to the fundamental voltage vector (1, 1, 0) is held for the time t.sub.2 and last, the switching pattern corresponding to the zero vector (1, 1, 1) is held for the time t.sub.0.
One of problems to be solved in the above-described inverter device is that further improvement in the degree of approximation of the voltage waveform to the sinusoidal wave by the PWM control has been desired. A second problem is that the magnetic flux and torque of the motor connected to the inverter device are biased in a unit region transition portion between the two kinds of fundamental voltage vectors in the prior art shown in FIGS. 18-22, which makes it difficult to stably control the motor. The second problem will be described in detail later. A third problem is that the waveform distortion of output voltage of the inverter device to which the voltage command value V* is supplied is increased since the magnitude of the inverter output voltage is .sqroot.3/2 times larger than the maximum PWM controllable voltage. More specifically, the principle in this control system is that a carrier voltage signal Va and a sinusoidal wave voltage signal Vb are compared and a PWM signal is obtained in accordance with the magnitude of each signal. When the amplitude of the sinusoidal wave voltage signal is excessively increased in this control system, the amplitude in the vicinity of its peak cannot be accurately converted to its corresponding time length, resulting in an increase in the waveform distortion.
The above second problem will be described in detail. Referring to FIG. 23, the phase command value .theta.*.sub.1 belongs to the unit region R.sub.1 and the phase command value .theta.*.sub.2 belongs to the unit region R2 with lapse of time. The magnetic flux vector is translocated with the increase in the phase as shown in FIG. 24. More specifically, the relation of t.sub.1 &gt;t.sub.2 holds with respect to the phase command value .theta.*.sub.1 belonging to the unit region R.sub.1 within the control period T.sub.SW where the output times of the switching patterns corresponding to the fundamental voltage vectors (1, 0, 0), (1, 1, 0) and the zero vector are represented as t.sub.1, t.sub.2, t.sub.0 respectively. Accordingly, the switching pattern corresponding to the zero vector (0, 0, 0) is first held for the time t.sub.0 and then, the switching pattern corresponding to the longer fundamental voltage vector (1, 0, 0) is held for the time t.sub.1 and last, the switching pattern corresponding to the shorter fundamental voltage vector (1, 1, 0) is held for the time t.sub.2.
The relation of t.sub.1 &lt;t.sub.2 holds when the phase command value .theta.*.sub.2 belongs to the unit region R.sub.2 in the subsequent control period T.sub.SW, as shown in FIG. 12. Accordingly, the switching pattern corresponding to the zero vector (1, 1, 1) is held for the time t.sub.0. The switching pattern corresponding to the longer fundamental voltage vector (1, 1, 0) is then held for the time t.sub.2 and last, the switching pattern corresponding to the shorter fundamental voltage vector (1, 0, 0) is held for the time t.sub.1. In this case the mean locus .psi.' of the magnetic flux vector figures a circular orbit as obvious from FIG. 24.
FIG. 25 illustrates a torque waveform where the magnetic flux vector moves as shown in FIG. 24. The torque waveform is increased when the phase leads an ideal state and decreased when the phase lags behind it. Accordingly, the torque is increased during the times t.sub.1, t.sub.2 of the formation state of the fundamental voltage vectors and decreased during the time t.sub.0 of the formation of the zero vector. Consequently, the torque takes such a distorted waveform as shown in FIG. 25 in the transition of the unit region, resulting in a large torque ripple.
FIG. 26 shows another method of selecting the fundamental voltage vectors in the transition of the unit region. When the transition of the unit region occurs with respect to the phase command value .theta.* belonging to the unit region R.sub.1 after the switching pattern corresponding to the fundamental voltage vector (1, 1, 0) is held for the time t.sub.2, the switching pattern corresponding to the fundamental voltage vector (1, 0, 0) is held for the time t.sub.1 with respect to the phase command value .theta.*.sub.2 belonging to the unit region R.sub.2 during the subsequent control period T.sub.SW. The switching pattern corresponding to the fundamental voltage vector (1, 1, 0) is then held for the time t.sub.2 and the switching pattern corresponding to the zero vector (1, 1, 1) is held for the time t.sub.0. The torque waveform in this control method is shown in FIG. 27. Although the distortion of the torque waveform can be improved, the distortion of the actual magnetic flux vector locus relative to an ideal means locus of the magnetic flux is increased instead, resulting in variations in the magnetic flux. Since the distortion of the torque or magnetic flux thus occurs both in the method in FIG. 24 and in the method in FIG. 26, these methods entail a problem in the stable control of the motor. Further, it is understood that the above-described problem can always be seen in the transition of the unit region positioned in the middle of two different fundamental voltage vectors.